1. Field of the Invention
The present invention relates to an image forming apparatus and a duplicator, such as a copier, a fax, a printer, etc., and in particular, to a sheet feed tray capable of accurately holding a leading end of the topmost surface of a stack bundle of envelop like recording or printing mediums at its leading end in a sheet feeding direction at a prescribed position of a separation or conveyance mechanism while providing fine quality of constant conveyance.
2. Discussion of the Background Art
Different from a case of a plain sheet, when plural envelope like printing mediums, such as a medicine envelop, a mailing envelope, etc., are stacked in a bundle state, the stacking height varies. For example, when plural envelopes each including a glue longitudinal margin at its widthwise center (e.g. a general envelop) are stacked in the bundle state, the center rises. When side corner of the envelope are accordion folded (e.g. a medicine envelope) to increase an inclusion amount, the thickness of each of the sides becomes twice larger than that of its center, so that a center of the envelope becomes extraordinary thin in comparison with the envelope sides when stacked in the bundle state. When a stacked bundle is depressed to decrease the thickness and accordingly thickness variation as well as the volume thereof, a performance of separating the envelopes deteriorates.
The Japanese Registered Patent No. 3,542,689 attempts such that a stack use bottom plate is longitudinally divided into plural pieces to be separately pushed up by springs, respectively, so that the topmost surface of a bundle of medicine envelopes pressure contacts a conveyance roller at the leading end in the sheet feeding direction. In such a medicine envelope feeder, to separate and convey the envelopes from the bundle one by one, the bundle needs to uniformly pressure contact the conveyance roller in the axial direction of the conveyance roller. However, in such a separate bottom lift up system, when a spring coefficient is different from others and the envelope bundle decreases, a height of each of the bottom plates gradually becomes different from the other. As a result, a pressure contact force of the envelope stack against the conveyance roller made by bottom plates becomes uneven, so that qualities of separation and conveyance of the stacked envelope deteriorate.
Further, due to the above-mentioned difficulty, the separate bottom lift up system can not employ a system widely used in a sheet feeding device of an electro-photographic image forming apparatus, in which a leading end of the topmost surface of a sheet bundle is held at a prescribed height and is separated and conveyed by a pick up roller and a sheet feed roller. Specifically, when plural sheet feeding cassettes each having the spring system bottom plate separation construction are piled up, and a stack height detection device is arranged in the vicinity of the pick up roller arranged almost at the widthwise center, the stack height of medicine envelopes becomes lower at the center than its both side ends. As a result, the both side ends contact the bottom plate of the sheet-feeding tray arranged above even the stack height detection device recognizes the height as being appropriate. Specifically, the envelope bundle sometimes causes a problem of separation and conveyance, such as deformation of the bottom plate, breakage of a lifting device, etc., in the worst case.